The present invention generally relates to an FM receiver which is capable of generating a signal showing the electric field strength of a received signal, and more particularly, to an FM receiver which is capable of generating a signal showing the electric field strength, wherein the dynamic range is expanded, and the linearity is improved.
Generally, the IF amplification stage of the FM receiver is composed of a multistage of differential type amplitude limit amplification circuit so as to improve an AM suppressing effect and a capture effect, with the gain of each stage being set from 6 dB to approximately 20 dB. Therefore, if a signal for showing the electric field strength from an optional stage of the IF amplification stage is taken out, the dynamic range will be narrowed from 6 dB to 20 dB or so.
As a method of enlarging the dynamic range of the signals for the electric field strength display, there is known such one as disclosed in Japanese Patent Publication Tokkosho No. 52-2561. As shown in FIG. 2, an FM RF signal received by an antenna 1 is amplified by an RF amplification circuit 2. Thereafter, the FM RF signal is mixed, by a mixer circuit 3, with an output signal of a local oscillation circuit 4, and is converted into an IF signal. The IF signal is amplified by the IF amplification circuits 5, 6 and 7 of three stages, and thereafter is FM-detected by an FM detection circuit 8. This method consists of this signal path, level detection circuits 9, 10 and 11 for level-detecting signals drawn from the IF amplification circuits 5, 6 and 7, and an addition circuit 12 for adding the output signals of the level detection circuits 9, 10 and 11, and is provided with an electric field strength signal generating circuit, for generating the output signals showing the electric field strength, at the output terminal of the addition circuit 12. In the case of the circuit of FIG. 2, the FM detection output signals may be obtained at a first output terminal 13. The electric field strength signal, whose dynamic range is enlarged, may be obtained at a second output terminal 14.
In the circuit of FIG. 2, as signals have to be obtained from a plurality of IF amplification circuits, a plurality of level detection circuits are required, thus resulting in complicated circuits. Undulation is caused in the electric field strength signal in the addition point so as to especially increase the number of the stages of the IF amplification circuit. When the electric field strength signal is so arranged as to be obtained for every other stage, a flat portion is caused in the addition point as shown in FIG. 4, thus making it impossible to effect the electric field strength display of good linearity. When the electric field strength signal is used as a stop signal of an automatic search, the stop signal detection sensitivity is likely to be influenced by the external circumstances if the stop sensitivity is set in the flat portion.